1. Field of the Invention
This invention relates to a bonding wire for use in forming bonded electrical connections for connecting leads to electrodes (pads) on a semiconductor chip, and more particularly to a bonding fine wire coated with insulating and abrasion-resistant layers.
A semiconductor chip is connected through bonding fine wires to leads which provide output terminals. For example, as shown in FIG. 2b, a circuit terminal (pad) 4 is provided on a semiconductor chip 3 mounted on a tab member 1 by an adhesive layer 2. This pad 4 is connected to an output terminal 5 by a bonding fine wire 6 the ends of which are bonded onto the pad 4 and the output terminal 5, respectively. The bonding fine wire is unreeled (not shown), guided by a roller 9 and clamper 8 etc. and reaches the end of the bore 10 of a tool (capillary) 7. At the end of the capillary 7, the leading end of the fine wire is heated into a melt and formed into a ball 11 by gas heating or arc heating etc. This ball is bonded onto a pad 4 through the capillary by thermocompression with or without ultrasonic vibration to make a bond (the first bond) 12. Subsequently, the fine wire is pulled out of the capillary 7 to reach an inner lead, and is bonded to the inner lead 5 (the second bond) with the aid of the end of the capillary 7 by thermocompression in combination with ultrasonic vibration.
A bonding fine wire of an electrically conductive metal, usually Au, Cu or Al, etc., is employed, with the fine wire assuming a certain loop height for connecting a pad to a lead. Because the fine wire is drawn from the heated end thereof on the pad through the capillary 7, the loop frequently sags, which sometimes results in a short circuit between the semiconductor chip and the fine wire. Particularly recently with the development of larger scale IC's involving the use of multiple pin structures, a tendency towards a longer distance between pad and lead has arisen. The use of a longer wire span for coping with this increased distance makes it easy for the fine wire to sag. This curling of the fine wire can cause unwanted contact to occur between the adjacent fine wires, which may result in short circuits being formed. The occurrence of defective electrical connections due to the short circuits formed by contact between adjacent fine wires because of the sagging and curling of these fine wires presents a significant problem.
As a countermeasure against such a problem encountered with the use of high-density, fine wires as above-mentioned, that is, for preventing short circuits between bonding fine wires or between fine wires and semiconductor elements, bonding fine wires covered over the surface thereof with an insulating film have been proposed, as for example, in Japanese Patent Laid-Open Application Nos. Sho. 58-3239 and 59-154054. Insulating materials disclosed in these patent specifications are of high molecular resins. When used, some of these resins may leave contaminating material such as carbon residue in the lower portion of the ball appearing during the aforesaid first bonding. This circumstance causes a poor bonding of the fine wire onto the pad.
Japanese Patent Laid-Open Application No. Sho. 63-318132 discloses a bonding process and apparatus intended to prevent a poor bond of the fine wire to the pad due to the presence of molten. resin at the ball, and/or the formation of a resin lump associated with the upward extension of melting during the heating of the leading end of the fine wire, wherein a fluid such as an inert gas is blow onto the leading end of the fine wire to cause molten resin to fly off and the molten resin is then removed by suction. The use of such an apparatus becomes too complicated, however, when it is to be attached to equipment for bonding a large number of fine wires for electrical connections, successively where the respective fine wires are to be closely spaced to each other.
Japanese Application Nos. Hei. 1-107067 and 114580 are directed to a bonding fine wire which has a good insulating quality, being coated with a resin selected from the group of aromatic polyesters and polycarbonate resins having specified physical properties, without using the above-mentioned complex apparatus.
Insulation-coated fine wires typically have the following characteristics:
(1) Insulating quality (Prevention against short circuits between the fine wire and a semiconductor chip or between respective fine wires caused by contact).
(2) Heat resistance (Prevention against the upward extension of melting from the ball at the leading end of the fine wire (upward peeling) and against melting of the insulating resin).
(3) Bonding property (Ability of the fine wire to be successively bonded, and to maintain bonding strength).
The resin-coated bonding fine wire described in the above-mentioned Japanese patent applications has these characteristics. However, a resin-coated bonding fine wire with these characteristics cannot be considered completely satisfactory in certain respects.
That is, in the bonding of resin-coated fine wire as described above, the coated wire touches bonding equipment when moving or when brought to a stop. For example, the resin coated fine wire may be clamped by a clamping member during movement or when brought to a stop; there may be contact by the resin-coated fine wire with the inside surface of the capillary while the resin-coated fine wire is being drawn from the first bonding to the second bonding; the association with ultrasonic vibration of the capillary for the second bonding may cause contact; and particularly, the pulling of the resin-coated fine wire by the clamping member for cutting the wire after completing the second bonding results in vigorous contact with the coated surface of the fine wire causing the resin coat to become flawed and to tear. When the surface state of the resin coat is damaged, its insulating function may be seriously impaired.
Thus, in spite of a desire for an insulating coating offering characteristics dealing with the above-mentioned types of situations, the manufacture of a coated fine wire satisfying problems associated with these situations has not been achieved.
It is the principal object of the present invention to provide a fine wire of an electrically conductive metal having a resin coating superior in insulating property, heat resistance, bonding property, and also abrasion resistance for use in forming a bonded electrical connection in a bonding operation of successive bonded connections without requiring a special apparatus having fluid blowing and suction inducing capabilities during the bonding operation, for example.
In order to achieve the above-mentioned object, a resin-coated fine wire for use in forming bonded electrical connections on a semiconductor device is provided in accordance with the present invention in which the resin coat includes a first layer of a resin having a good insulating property and heat resistance and a second layer of a resin having good abrasion resistance over the first layer.
In another aspect of the invention, resin-coated fine wire for use in forming bonded electrical connections on a semiconductor device is provided in which a composite resin coating covering the fine wire of electrically conductive metallic material includes a first coating layer made of at least one resin selected from the group consisting of aromatic polyester resins, such as polyarylate resins and polycarbonate resins, and a second coating layer made of at least one resin selected from the group consisting of polyurethanes, polyester imides, and polyimides.
In a further aspect of the invention, a resin-coated fine wire for use in forming bonded electrical connections on a semiconductor device is provided in which the resin coating includes a first resin coating layer 0.1 to 1.2 .mu.m thick and a second resin coating layer 0.1 to 1.0 .mu.m thick.